1. Technical Field
This invention relates to nickel base superalloy single crystal castings or articles such as turbine engine blades and vanes and a method for densification of such castings.
2. Background Art
Single crystal nickel base superalloys are cast to produce articles having superior strength characteristics at elevated operating temperature. Due to the nature of the alloy and method of casting, these castings contain porosity on the order of 0.2 to 0.5% by volume. These pores have been found to be the primary initiation site for cracks which lead to fatigue failure. Removal of these pores lends a significant improvement to the component's life cycle. Conventional hot isostatic pressing (HIP) techniques have been applied to single crystal components without success.
Standard HIP pressures of about 15,000 psi are adequate to eliminate porosity but result in internal recrystallization which negates much of the benefits. HIP temperature control has been found to be critical. At temperatures below the accepted solution heat treatment range, the strength of the material resists the deformation necessary for closure of pores and the thermal energy is insufficient to bond the closed pores.
Single crystal superalloys are also prone to incipient melting, the localized melting of second phases in the microstructure, at temperatures above the accepted solution heat treatment range. The acceptable solution heat treatment range of cast superalloy single crystals can be as narrow as approximately 5.degree. F. while a typical HIP vessel temperature range may vary up to about 20.degree. F.
For optimum properties, single crystal superalloys must be cooled from the solution heat treatment temperature at rates greater than those achievable in production HIP vessels. This requires a post-HIP solution heat treatment to achieve the desired cooling rate. Pores closed by HIP have been found to reopen during standard solution heat treatment due to the diffusion of alloying elements.